In an optical transmission system, total harmonics distortion (THD) is one factor of deterioration of a signal-to-noise ratio (SNR). The total harmonics distortion indicates a ratio of a harmonics component to a fundamental wave component. Namely, the total harmonics distortion THD is expressed according to formula (1).
                    THD        =                                                                              ∑                  1                  n                                ⁢                                                                  ⁢                                  P                  n                                                            P                0                                              ×          100                                    (        1        )            P0 indicates the power of a fundamental wave. Pn (n=1, 2, . . . ) indicates the power of each harmonics.
FIG. 1 illustrates an example of an optical transmission system. The optical transmission system includes a modulation circuit 1, an optical transmitter 2, an optical transmission link 3, an optical receiver 4, and a demodulation circuit 5. In this example, the optical transmission system transmits an optical discrete multi-tone (DMT) signal. Accordingly, the modulation circuit 1 includes a DMT modulator and a digital-to-analog (D/A) converter. The optical transmitter 2 includes a driver and a light source circuit (LD). The driver generates a drive signal from an output signal of the modulation circuit 1. The light source circuit generates an optical DMT signal based on the drive signal. The optical DMT signal is transmitted to the optical receiver 4 via the optical transmission link 3. The optical receiver 4 includes a photo detector (PD) and an amplifier (TIA). The photo detector performs optical-to-electrical conversion so as to convert the optical DMT signal into an electric signal. The amplifier amplifies the electric signal that is output from the photo detector. Alternatively, the amplifier converts a current signal that is output from the photo detector into a voltage signal. The demodulation circuit 5 includes an analog-to-digital (A/D) converter and a DMT demodulator. Total harmonics distortion may be generated in the optical transmitter 2 and the optical receiver 4.
In the optical transmission system, the total harmonics distortion needs to be estimated accurately in order to improve the quality (for example, an SNR) of a received signal. In the configuration illustrated in FIG. 1, it is preferable that the total harmonics distortion be estimated in each of the optical transmitter 2 and the optical receiver 4.
FIG. 2 illustrates an example of a method for estimating total harmonics distortion generated in the optical receiver 4. In the example illustrated in FIG. 2, a total harmonics distortion estimator includes an oscillator 11, an electrical-to-optical (E/O) circuit 12, and an RF spectrum analyzer 20. The oscillator 11 oscillates at a frequency that corresponds to a given control signal, and generates a sine wave signal. Namely, the oscillator 11 generates a sine wave signal having a frequency that corresponds to the control signal. The E/O circuit 12 generates an optical sine wave based on the sine wave signal that is output from the oscillator 11. As an example, the E/O circuit 12 modulates carrier light by using the sine wave signal so as to generate an optical sine wave. The optical sine wave is used to estimate total harmonics distortion of an estimation target. Accordingly, in the example illustrated in FIG. 2, the optical sine wave is input into the optical receiver 4. The RF spectrum analyzer 20 detects a spectrum of an electric signal that is output from the optical receiver 4. In the example illustrated in FIG. 2, a fundamental wave component f0 and its harmonics components f1 and f2 are detected. f0 corresponds to an oscillation frequency of the oscillator 11.
Total harmonics distortion is calculated according to the formula (1). Accordingly, frequency dependency of the total harmonics distortion generated in the optical receiver 4 can be obtained by sweeping the oscillation frequency of the oscillator 11.
A method for measuring total harmonics distortion generated in an optical receiver is described, for example in Document 1. In addition, the specification of a request relating to total harmonics distortion of a component that configures a DMT transmission system is described, for example, in Document 2. Further, influence of total harmonics distortion on signal quality of pulse amplitude modulation (PAM) is described, for example, in Document 3.    Document 1: Lian Zhao et al., “10G Linear TIA in Long-reach Multi-mode Applications,” inphi corp. (http://www.mpdigest.com/issue/Articles/2008/Japan/inphi/)    Document 2: David Lewis et al., “400G DMT PMD for 2 km SMF,” JDSU    Document 3: Francois Tremblay, “PAM-8 and PAM-16 Optical Receivers for 2 km 100G Links with a 4 dB loss budget.” (http://www.ieee802.org/3/100GNGOPTX/public/mar12/plenary/)
In considering a scheme for suppressing total harmonics distortion of an optical transmission system, it is preferable that total harmonics distortion generated in a transmitter side and total harmonics distortion generated in a receiver side be estimated individually. However, in a conventional technology, it is difficult to accurately estimate total harmonics distortion generated in an optical receiver. In the configuration illustrated in FIG. 2, for example, total harmonics distortion may be generated in the E/O circuit 12. Therefore, a modulated optical signal (in the example above, an optical sine wave) that is input into the optical receiver 4 may have a large total harmonics distortion. In this case, total harmonics distortion detected based on an output signal of the RF spectrum analyzer 20 includes total harmonics distortion generated in the E/O circuit 12 and total harmonics distortion generated in the optical receiver 4. Accordingly, it is not easy to accurately estimate the total harmonics distortion generated in the optical receiver 4.